1. Field of the Invention
The present invention relates to an optical system and a photographic device including the optical system.
2. Description of the Related Art
Generally, with the optical system applied to the photographic device such as the digital camera or the video camera, the more an overall length of the lens system (overall optical length, i.e., length between a surface of a first lens on an object side and an image plane) is shortened to downsize the entire optical system, the more aberrations, for example longitudinal chromatic aberration and lateral chromatic aberration, are generated, thus degrading the optical performance. In particular, a telephoto type optical system, in which the overall length of the optical system is shortened, causes more chromatic aberrations to be generated such as when a focal length is lengthened.
For reducing the generation of such chromatic aberrations, there is generally known a method using an extraordinary partial dispersion material for an optical material, and a method using a diffractive optical element in an optical path.
In the telephoto type optical system, the chromatic aberration is generally reduced by using a lens having a positive refractive power composed of a low dispersion optical material having extraordinary partial dispersion such as a fluorite and a lens having a negative refractive power composed of a high dispersion optical material, by respectively disposed in a front lens group, at which a paraxial marginal ray and a paraxial chief ray pass through positions relatively higher than a position of an optical axis (refer to Japanese Patent Publication No. 60-49883, which corresponds to U.S. Pat. No. 4,241,983; Japanese Patent Publication No. 60-55805, which corresponds to U.S. Pat. No. 4,348,084; and Japanese Patent Laid-Open No. 11-119092, which corresponds to U.S. Pat. No. 6,115,188).
The paraxial marginal ray, by the definition when the focal length of the entire optical system is normalized to 1, is a paraxial ray incident in parallel to the optical axis of the optical system with a height of 1 from the optical axis. The paraxial chief ray, by the definition when the focal length of the entire optical system is normalized to 1, is a paraxial ray passing an intersection point of an entrance pupil and the optical axis of the optical system from among rays incident by −45° relative to the optical axis. The incident angle of the ray to the optical system is assumed to be positive in the clockwise direction, or negative in the counterclockwise direction when measured relative to the optical axis. Assume that an object is on the left side of the optical system, and the ray incident on the optical system from the object side is advanced to the right side from the left side.
Alternatively, there is known an optical system which corrects and/or reduces the chromatic aberrations in the telephoto type optical system with the use of the diffractive optical element without use of the optical material having the extraordinary partial dispersion. Japanese Patent Laid-Open No. 6-324262, which corresponds to U.S. Pat. No. 5,790,321; and Japanese Patent Laid-Open No. 6-331887, which corresponds to U.S. Pat. No. 5,629,799 discuss a telephoto type optical system with a F-number of about F2.8, the optical system correcting and/or reducing the chromatic aberration relatively reliably by combining the diffractive optical element and a refractive optical element.
Still alternatively, a liquid-state material having a relatively high dispersion characteristic and a relatively extraordinary partial dispersion characteristic is known as an optical material capable of correcting and/or reducing the chromatic aberration, the characteristic being related to the optical characteristic of the diffractive optical element, and an achromatic optical system using the liquid-state material is suggested in U.S. Pat. No. 5,731,907 and U.S. Pat. No. 5,638,215.
The telephoto type optical system discussed in Japanese Patent Publication No. 60-49883, which corresponds to U.S. Pat. No. 4,241,983; Japanese Patent Publication No. 60-55805, which corresponds to U.S. Pat. No. 4,348,084; and Japanese Patent Laid-Open No. 11-119092, which corresponds to U.S. Pat. No. 6,115,188 using the fluorite for the optical material can easily correct the chromatic aberration when the overall length of the lens system is set relatively long.
However, the shorter the overall length of the lens system is set, the more the chromatic aberrations are generated; it can be difficult to reliably correct the chromatic aberrations. Because the method merely reduces the chromatic aberration generated in the front lens system having the positive refractive power by using the low dispersion characteristic and the extraordinary partial dispersion characteristic owned by the material such as the fluorite. When the chromatic aberration, which increases along with the shortening of the overall length of the lens system, is intended to be reduced, in a case of a lens for instance using a low dispersion glass with a large Abbe number such as the fluorite, the chromatic aberration can not remarkably be changed if the refractive power of a lens surface is largely changed.
Accordingly, the correction and/or reduction of the chromatic aberration is hard to be correlated with correction of other aberrations such as spherical aberration, coma and astigmatism generated when the refractive power is largely changed.
Meanwhile, as the diffractive optical element has a sufficient correction effect for the chromatic aberration, it however, generates a diffractive light having an unnecessary order of diffraction, which is not a designated order of diffraction.
Since the material discussed in U.S. Pat. No. 5,731,907 and U.S. Pat. No. 5,638,215 is in liquid state, a structure for sealing the liquid is necessary, and is difficult to be manufactured in the case of application to the optical material. In addition, a characteristic such as a refractive index and dispersion can be varied with temperature, thus resulting in insufficient environment resistance. Further, since the Abbe number is relatively large and the extraordinary partial dispersion is relatively small, as well as a boundary for the air is not provided, a sufficient correction effect of the chromatic aberration is hardly obtained.